1. Field of the Invention
The present invention relates generally to gas fired water heating appliances, and more particularly, but not by way of limitation, to relatively high capacity water heating appliances having a continuously variable burner input over a wide input range thus providing a relatively high turndown ratio as compared to prior systems.
2. Description of the Prior Art
Conventional water heating appliance technology utilizes a burner designed to operate at a fixed flow rate of combustion air and fuel gas to the burner. Such an appliance cycles on and off in response to a control system which monitors the temperature of heated water in a storage tank or elsewhere in various conduits of the water supply system. One example of such a typical prior art system which is presently being marketed by the assignee of the present invention is that shown in U.S. Pat. Nos. 4,723,513 and 4,793,800 to Vallett et al., the details of which are incorporated herein by reference.
It has been recognized that, in circumstances where there is a substantially varying demand for heat input to the water supply system, greater energy efficiencies can be achieved through the use of a water heating appliance which is capable of operating at different energy inputs. One example of such a system is that sold by Lochinvar Corporation, the assignee of the present invention, under the trademark COPPER-FIN II®. The Lochinvar COPPER-FIN II® system utilizes a plurality of staged burners which can be brought on line or taken off line as the demand for heat energy changes. The COPPER-FIN II® appliance includes multiple banks, for example, first, second, third and fourth stages. It initially turns on all four stages of burners, and as it approaches the desired temperature, it sequentially shuts off units to decrease input energy. This type of system provides variable input, but it is not continuously variable. Instead the input can be changed only in substantial increments corresponding to the heat input of one burner stage.
The prior art has also included proposals for water heaters having continuously variable input over a range of inputs. Two such systems are shown in U.S. Pat. No. 4,852,524 to Cohen and U.S. Pat. No. 5,881,681 to Stuart. These systems, which have been marketed by AERCO International, Inc. under the Benchmark name, utilize a nozzle mix burner which receives independent streams of combustion air and fuel gas. A fuel/air valve is utilized to electronically and simultaneously control the flow of air through the air line and fuel through the fuel line so as to provide a varying input of fuel and air while maintaining a constant fuel to air ratio. The blower speed remains constant on these systems. The Stuart U.S. Pat. No. 5,881,681 patent suggests that the system described therein can achieve turndown ratios as high as 15:1. AERCO's advertising literature for its Benchmark model water heaters suggests that they achieve turndown ratios as high as 20:1.
More recently the assignee of the present invention has developed a continuously variable water heating appliance with variable air and fuel input, as shown in U.S. Pat. No. 6,694,926 to Baese et al. In the Baese apparatus a variable flow blower provides premix combustion air and fuel to the burner at a controlled blower flow rate within a blower flow rate range. This allows the heat input of the water heating appliance to be continuously varied within a substantial flow range having a turndown ratio of as much as 4:1.
Inherent physical limitations on the turndown ratio which can be achieved with a single heating apparatus of prior designs makes it difficult to achieve a continuous range of heat input over a large operating range from a very low low end for low heat demand situations to a very high high end for high heat demand situations. One prior solution to this difficulty is to utilize a plurality of commonly controlled heat exchangers such as those of the Baese et al. patent described above. One such system is described for example in U.S. Patent Application Publication No. 2008/0216771 of Paine et al., and assigned to the assignee of the present invention. While such multiple modulating systems do solve the problem of providing continuous modulation over a wide range of heat demands, they do so at the cost of increased complexity of plumbing to connect the multiple units and increased complexity of control systems to coordinate the operation of the units.
Thus there is a continuing need for a relatively large capacity single unit heating apparatus which can provide continuous modulation of heat input over a wide range of heat demands.